Circuit arrangement and method for operating a high-pressure discharge lamp

ABSTRACT

A circuit arrangement for operating a high-pressure discharge lamp ( 12 ) with an electronic ballast, which is designed to provide an AC feed signal for the high-pressure discharge lamp ( 12 ). The AC feed signal comprises, in the time range as modulation period (T), a serial sequence of at least one first signal section (Sa 1 ), one second signal section (Sa 2 ) and one third signal section (Sa 3 ), which are associated with a first, a second and a third color. The AC feed signal is an amplitude-modulated RF signal (I RF ) with a frequency (f) of at least 500 kHz. The following applies for the envelope (E) of the signal components with a positive and/or negative amplitude: the absolute value of the envelope (E) has a mean amplitude (M); and the first signal section (Sa 1 ) and/or the second signal section (Sa 2 ) and/or the third signal section (Sa 3 ) has/have at least one temporal range in which the absolute value of the envelope (E) is between 5% and 100% greater than the mean amplitude and/or in which the absolute value of the envelope (E) is between 5% and 90% less than the mean amplitude (M).

RELATED APPLICATIONS

This is a U.S. national stage of application No. PCT/EP2006/069812,filed on Dec. 18, 2006.

FIELD OF THE INVENTION

The present invention relates to a circuit arrangement for operating ahigh-pressure discharge lamp with an electronic ballast, which isdesigned to provide an AC feed signal for the high-pressure dischargelamp, which AC feed signal comprises, in the time range as modulationperiod, a serial sequence of at least one first signal section, onesecond signal section and one third signal section, which are associatedwith a first, a second and a third color. The invention furthermorerelates to a projection apparatus with a corresponding circuitarrangement and to a corresponding method for operating a high-pressuredischarge lamp.

BACKGROUND OF THE INVENTION

High-pressure discharge lamps as are used, for example, as videoprojection lamps, generally have two identical electrodes, which areusually in the form of rods. Very disruptive flicker phenomena may ariseduring operation of such high-pressure discharge lamps on alternatingcurrent. These flicker phenomena arise owing to alternating jumping ofthe root of the arc onto the electrode peaks. This is made possible bythe electrode function changing frequently from the anodic phase(positive polarity) to the cathodic phase (negative polarity) at theoperating frequency. Such jumping of the arc root in particular impairs

the use of high-pressure discharge lamps in optical devices, for exampleprojection devices, video projectors, microscope lighting and can evenresult in it not being possible to use these lamps in this application.

U.S. Pat. No. 5,608,294 has disclosed, for low-frequency (50 Hz to a few100 Hz) operation of a high-pressure discharge lamp, superimposing shortsynchronous pulses on the square-wave lamp current profile forstabilization purposes, i.e. in order to prevent the root of the arcfrom jumping. In this case, the current at the end of a half period isincreased temporarily prior to subsequent commutation. In accordancewith the mentioned document, the current pulse prior to the commutationresults in a temporary increase in temperature at the current-conductingroots of the arc on the electrodes, primarily the anode at that time.This results in a material deposition (electrode reformation), i.e. theelectrode metal tungsten from the gas cycle process is deposited on theelectrodes from the tungsten halides, and a peak is formed on theelectrodes, which stabilizes the discharge and the root of the arc veryeffectively.

WO 03/098979 A1 has disclosed the operation of a high-pressure dischargelamp with an unmodulated RF signal of more than 3 MHz. In general,high-pressure discharge lamps permit successful RF operation only abovefrequencies which are above the acoustic resonances in the combustionchamber. These acoustic resonances result in strong flows in thecombustion chamber which generally considerably disrupt the dischargearc. However, the literature contains attempts to damp

the acoustic resonances by means of suitable feed currents or tocompletely avoid said resonances. By way of example, reference is madeto DE 10 2005 028 417.5 and DE 10 2005 059 763.7. Such solutions areusually very complex, however.

Finally, reference is made to DE 198 29 600 A1, which is concerned withRE operation of a high-pressure discharge lamp. It relates in particularlikewise to the problem of the jumping of the root of the arc onto theelectrode peaks. Against the background of a prior art in which thehigh-pressure discharge lamps were operated at a frequency of below 2kHz, said document proposes the solution of operating the lamp at afrequency above 800 kHz, preferably above 1 MHz and particularlypreferably between 2 and 3 MHz. In a preferred development, theoperating frequency is wobbled both continuously and suddenly with amodulation frequency of less than 10 kHz, preferably between 1 and 2kHz. Although this can under some circumstances provide a solution forcertain high-pressure discharge lamps, this measure has proven to beineffective in the case of the high-pressure discharge lampsinvestigated by the inventors of the present invention.

The basic solution of preventing jumping of the root of the arc onto theelectrode peaks during RF operation of a high-pressure discharge lamp isprovided in the subsequently published patent applicationPCT/EP2006/068269 by the same applicant as the present application. Thesolution consists in the electronic ballast further being designed tomodulate the AC feed signal in terms of its amplitude.

The present application is aimed at a preferred use sector of suchhigh-pressure discharge lamps: the known term DLP (digital lightprocessing) is used to describe a technology which is used in videoprojectors and rear-projection televisions. It is based onmicroscopically small mirrors which are fitted on a DMD (digitalmicromirror device) chip. In this case, the mirrors are smaller than afifth of the width of a human hair. They have two stable end states,between which they can alternate within 16 μs in a preferred embodiment.The movement is brought about by the force effect of electrostaticfields. Owing to the incline of the individual micromirrors on the DMDchip, the light is either reflected directly towards the optical unit ordirected towards an absorber. By pulse-width-modulated driving of themirrors, various brightness levels of the individual pixels can begenerated.

DMD chips with an XGA image resolution of 1024×768 contain anarrangement of 786,432 tiny mirrors. In the meantime, DMD chips withresolutions of up to 2048×1080 can be obtained, i.e. approximately twomillion mirrors.

Since the DMD chips reflect the white light of a projection lamp,additional steps are required for a colored image. In a 1-chipprojector, a color wheel is connected into the optical path in front ofthe DMD chip, with color filters of the primary colors (generally thecolors red, green and blue, but sometimes also other colors as well)rotating on said color wheel. In order to achieve improved brightnessvalues in the white region, white is also added to the color wheel. Withthe position of the color filter, the electronics change

the partial image which is reflected by the DMD. Owing to the rotationalspeed of the color wheel and the inertia of the human eye, the partialimages are added to form a colored image impression. Since the detectionfrequency is different from human to human, there were reports,primarily in the case of the first models, of a so-called rainboweffect, which occurred when the viewer perceived the individual colors.In a further step, the revolution number of the wheel was thereforedoubled and the number of color segments increased in the case of morerecent models.

The basic design of such a projection apparatus is provided, forexample, in U.S. Pat. No. 5,917,558. FIG. 2 of said document U.S. Pat.No. 5,917,558 shows various pulse control modes for the projection lamp.As can be seen from said figure, these pulses are LF pulses, with amodulation period comprising a serial sequence of a plurality of signalsections which are associated with different colors, in a time range. Ifa high-pressure discharge lamp is used as a projection lamp,unfortunately the abovementioned undesirable effect of the jumping ofthe root of the arc onto the electrode peaks occurs during operationwith such pulse trains.

Moreover, with such operation, a noticeable dip in the luminous flux andtherefore a loss of control of the luminous flux in this period arisedespite the rapid commutation of the lamp. This loss of control needs tobe avoided in present-day applications by this period being placed inblanking intervals. Furthermore, the procedure in accordance with theprior art displays oscillation phenomena of the luminous flux after thedip in the luminous flux. In this period, the luminous flux cantherefore not be controlled and often also cannot be used. The loss ofcontrol in modern-day applications disrupts, for example, the colorbalance and needs to be compensated for by complicated measures in thedevice.

Further prior art can be found in U.S. Pat. No. 5,109,181, DE 100 18 860A1 and US 2006/0022613 A1.

SUMMARY OF THE INVENTION

One the object of the present invention is to provide the circuitarrangement mentioned at the outset or the method mentioned at theoutset in which jumping of the root of the arc onto the electrode peaksis reliably prevented.

This object can be achieved if an amplitude-modulated RF signal with afrequency of at least 500 kHz is initially used as the AC feed signal.For reasons associated with the EMC, a frequency of approximately 50 MHzis preferably selected as the upper frequency range limit. Moreover, theamplitudes of the signal sections associated with the individual colorsshould differ from one another such that the effect already mentioned inPCT/EP2006/068269 is produced. In very general terms this means that thefollowing applies for the envelope of the signal components with apositive and/or negative amplitude:

-   -   the absolute value of the envelope has a mean amplitude;    -   the first signal section and/or the second signal section and/or        the third signal section has/have at least one temporal range in        which the absolute value of the envelope is between 5% and 100%        greater than the mean amplitude and/or in which the absolute        value of the envelope is between 5% and 90% less than the mean        amplitude.

With the procedure according to an embodiment of the invention, it ispossible for the emitted luminous flux of the lamp to be subjected touninterrupted control. This control of the luminous flux takes placevery quickly with an extremely short delay and can have high dynamics ofthe luminous flux modulation. As a result of the extremely quick,uninterrupted control of the luminous flux emitted by the lamp, nointerruptions to the luminous flux or transient phenomena occur whichcan barely be avoided during operation in accordance with the prior artowing to the commutation of the lamp current. The control of theluminous flux can take place in a very simple manner via the level ofthe amplitude of the feeding RF current in the case of the invention.

The circuit arrangement according to an embodiment of the inventionmakes it possible to control the luminous flux extremely quickly with avery great modulation depth. As a result, both high dimming dynamics andoverbrightening dynamics can be achieved. In addition to the capabilityof following for example rapidly successive image contents with thecorresponding brightness, this is of high importance when preciselymixing the colors with different lamp brightnesses in individual colorwheel sectors. Only the possibility of precise dimming oroverbrightening of the lamp in the individual color wheel sectorswithout oscillation phenomena and without spectral reaction which isprovided by a circuit arrangement according to the invention allows forextremely precise mixing of colors.

The reasons for which an immediate (<1 s), effective stabilization ofthe arc during radiofrequency operation can be achieved in principle byan AC feed signal, formed as an amplitude-modulated RF signal with afrequency of at least 500 kHz, are not yet entirely explained at presentsince, in addition to increases in amplitude, as mentioned, reductionsin amplitude also bring about the success in accordance with theinvention and result in the avoidance of flicker phenomena of dischargearcs quite generally, and in particular plasma arcs in high-pressuredischarge lamps. An indication of this is the fact that a stabilizingpeak formation similar to that which results during operation with thecircuit arrangement proposed in the mentioned U.S. Pat. No. 5,608,294,is not set until after a few hours. In other words, this means that theactual explanation for the solution according to the invention cannoteven be found, or at least not only, in the peak formation.

However, and this is the most important aspect, stabilization can thusbe achieved as regards time constancy and location constancy of thedischarge arc, and this

stabilization satisfies even the stringent optical requirements placedon projection lamps.

In a preferred embodiment, in this case the AC feed signal is asymmetrically amplitude-modulated RF signal, where the absolute value ofthe envelope of the signal components with a positive amplitude is equalto the absolute value of the envelope of the signal components with anegative amplitude. In an alternative preferred embodiment, the AC feedsignal comprises an RF signal and an LF signal with varying amplitude.In this case, the envelope of the RF signal can have a constantamplitude, but it can also have a varying amplitude, where the variationis matched to the variation of the amplitude of the LF signal. Preferredmatching can be designed such that an increased amplitude of the LFsignal is compensated for by a reduced amplitude of the envelope of theRF signal. As a result, the high-pressure discharge lamp emits aluminous flux with a constant amplitude and is deenergized duringsubstantially shorter periods of time in comparison with the prior art.As a consequence, no flicker phenomena can be perceived by an observer,but reformation of the electrodes takes place, as in the prior art. In apreferred alternative matching process, the amplitude of the envelope ofthe RF signal and the amplitude of the LF signal can be selected suchthat a compromise is found between a luminous flux which is as constantas possible and a sufficient electrode reformation.

In general, the absolute value of the envelope of at least one of thesignal sections preferably comprises a range of a first amplitudefollowed by at least one range of a second amplitude, where the absolutevalue of the second amplitude is less than the absolute value of thefirst amplitude. In this case, the second amplitude is preferably from50 to 90%, more preferably 67%, of the first amplitude.

Moreover, a range of a third amplitude can be provided, with the secondamplitude being from 50 to 90%, preferably 67%, and the third amplitudebeing from 2 to 50%, preferably 37%, of the first amplitude.

Particularly preferably, the electronic ballast has a control loop forcontrolling the RF power emitted to the high-pressure discharge lamp. Inthe LF circuits used in the prior art, i.e. circuit arrangements whichprovide an LF signal as the AC feed signal, the feed circuit is measuredfor this purpose and, from this, a conclusion is drawn regarding theemitted power. However, in circuit arrangements according to theinvention which provide an amplitude-modulated RF signal as the AC feedsignal, this is unfavorable owing to the variation in the efficiency, inparticular as a result of its dependence on the temperature, in acircuit arrangement according to the invention. The control looptherefore preferably comprises an apparatus for determining the actualvalue of the RF power and a setpoint input apparatus for inputting thesetpoint value for the RF power.

Preferably, the apparatus for determining the actual value of the RFpower comprises an apparatus for fixing the RF current emitted to thehigh-pressure discharge lamp, an apparatus for fixing the RF voltagepresent across the high-pressure discharge lamp, and an apparatus fordetermining the actual value of the RF power from the RF current and theRF voltage, in particular by analog linking of the RF current and the RFvoltage. In particular in the case of the last-mentioned analog linkingof the RF current and the RF voltage, the actual value of the RF powercan be determined virtually directly without the indirect route ofdigital calculation. This allows for a control loop which is as quick aspossible.

Preferably, the apparatus for fixing the RE current emitted to thehigh-pressure discharge lamp comprises a first peak-value rectifier, andthe apparatus for fixing the RF voltage present across the high-pressuredischarge lamp comprises a second peak-value rectifier.

Preferred embodiments of the apparatus for determining the actual valueof the RF power represent, for example, a ring mixer or a bridge mixer.In principle, it is thereby possible to realize closed-loop controlwhich is as quick as is allowed by the plasma in the high-pressuredischarge lamp.

In a preferred development of the circuit arrangement according to theinvention, this circuit arrangement furthermore comprises afrequency-dependent, in particular resonant, load network, whichprovides the RF current and the RF voltage to the high-pressuredischarge lamp depending on the driving frequency, the control loopfurthermore comprising an actuator for determining a change in thefrequency driving the load network from the difference between thesetpoint value and the actual value of the RF power. This allows forparticularly rapid closed-loop control.

Particularly advantageous is a design of the control loop whichimplements the control in a color-specific manner, i.e. separately forat least the first, the second and the third color.

The setpoint input apparatus can be designed to vary the setpoint valueto be input to the control loop temporally corresponding to the presentcolor and/or in order to provide reduced light intensities. This makesit possible, with limited stepping of the light valve, to halve orquarter the step amplitude and therefore to double or quadruple theresolution.

Finally, the setpoint input apparatus can have an interface, via whichat least one setpoint value can be changed, in particular by a user.

An aspect of the present invention relates to a projection apparatuswith a circuit arrangement according to an embodiment of the invention.Such a projection apparatus allows for much improved utilization of theenergy emitted to the projection lamp and therefore results in a higherdegree of efficiency than in the prior art. This allows the fansprovided in such projection apparatuses to be given markedly smallerdimensions, which results in a very desirable reduction in the noiselevel during operation of such projection apparatuses. The mentionedadvantages result from the fact that, when using a circuit arrangementaccording to the invention, no commutation intervals need to be providedin the profile of the current driving the high-pressure discharge lamp.In the prior art, the light valve needed to be driven, in order to avoidlight faults during commutation, in such a way that the light isdirected away from the lens and therefore does not arrive at thedisplay.

The preferred embodiments mentioned with reference to the circuitarrangement according to the invention and with reference to theprojection apparatus according to the invention and the advantagesthereof apply correspondingly, if applicable, to the method according tothe invention.

BRIEF DESCRIPTION OF THE DRAWING(S)

Exemplary embodiments of the invention will now be described in moredetail below with reference to the attached drawings, in which:

FIG. 1 shows a schematic illustration of the design of a projectionapparatus according to an embodiment of the invention;

FIG. 2 shows a schematic illustration of the design of a control loopused in the illustration in FIG. 1 for controlling the RF power emittedto the high-pressure discharge lamp;

FIG. 3 shows a first exemplary embodiment of an apparatus fordetermining the actual value of the RF power used in the illustration inFIG. 2;

FIG. 4 shows a second exemplary embodiment of an apparatus fordetermining the actual value of the RF power used in the illustration inFIG. 2;

FIG. 5 shows a third exemplary embodiment of an apparatus fordetermining the actual value of the RF power used in the illustration inFIG. 2;

FIG. 6 shows a schematic illustration of the time profile of an AC feedsignal for the high-pressure discharge lamp in accordance with a firstexemplary embodiment;

FIG. 7 shows a schematic illustration of the time profile of theluminous flux associated with the time profile of the RF current in FIG.6;

FIG. 8 shows a schematic illustration of the time profile of an AC feedsignal for the high-pressure discharge lamp in accordance with a secondexemplary embodiment; and

FIG. 9 shows a schematic illustration of the time profile of an AC feedsignal for the high-pressure discharge lamp in accordance with a thirdexemplary embodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of the design of an exemplaryembodiment of a projection apparatus 10 according to the invention. Thisprojection apparatus has a high-pressure discharge lamp 12, whoseradiated light is directed from a reflector 14 onto a color wheel 16.The color wheel 16 comprises three 120° segments of dichroitic filtersof different colors, in this case red, green and blue. The color wheel16 can rotate about the axis and is driven by a wheel drive system 18,which produces an output signal 20 which is representative of theposition of the color wheel 16 and consequently of the relevant colorwhich is in the output beam 21 of the high-pressure discharge lamp 12.The output beam 22 after the color wheel 16 is in this case sequentiallyred, green and blue. The output beam 22 is directed through a beamsplitter 23 onto the surface of a light valve 24, which in this exampleis a deformable mirror device. The impinging beam 22 of colored light ismodulated corresponding to the video information provided by the lightvalve 24 by means of a light valve controller 26, which receives thevideo information from a video signal input 28. The modulated beam ofcolored light which is reflected on the surface of the light valve 24 isfocused on a mirror 32 by a projection lens 30 and is reflected towardsa reproduction screen 34. During operation, each color is projectedsequentially onto the light valve, is modulated by the light valve 24with the specific video information for this color and projected ontothe screen 34. The colored images follow one another so quickly that theeye integrates the individual images to form a complete colored image.

The information on the position of the color wheel is provided via aline 35 to a control loop 38 and the valve controller 26, which,together with an apparatus 40 for determining the actual value of the RFpower, is combined in a circuit arrangement 36. This circuit arrangement36 furthermore comprises an interface 42, via which a user has theoption of setting the relative component of the colors in the outputsignal of the apparatus 40 for measuring the RF power with which thehigh-pressure discharge lamp 12 is driven, for example in aregion-specific manner.

FIG. 2 shows a more detailed illustration of the control loop 38 in FIG.1 in accordance with a preferred exemplary embodiment. It has a setpointinput apparatus 44 which inputs a setpoint value P_(set) for the RFpower to a differentiating apparatus 46, which moreover is supplied theactual value P_(act) of the RF power. This differentiating apparatusdetermines the control discrepancy ΔP and supplies this to an actuator48, which in this case comprises a unit 50 in order to determine amanipulated variable ΔU from the control discrepancy ΔP. The block 50can reproduce a look-up table or a formulaic relationship between ΔP andΔU.

The controlled variable ΔU is supplied to a control path 52, whichprovides the RF signal driving the high-pressure discharge lamp 12 atits output. In this case, the control path 52 comprises a VCO (voltagecontrolled oscillator) 54. At its output, the VCO 54 provides,corresponding to the changed voltage at its input, a signal with achanged frequency f, which is supplied to a control unit 56 in order togenerate the signal, via the driving of a switch S1, which signal isused to drive the high-pressure discharge lamp 12. The radiofrequencyswitching stage which in this exemplary embodiment is in the form of aclass E switching stage with zero voltage switching, comprises a DCvoltage source U_(dc), an inductance L_(dc), a diode D1 and a capacitorC1, in addition to the control apparatus 56. As is obvious to a personskilled in the art, other designs of the RF switching stage can beprovided, for example in the form of a push-pull RF switching stage withzero voltage switching, in the form of an RF half bridge with zerovoltage switching or the like.

The control path 52 furthermore comprises a resonant load filter 60,which in this case is intended to enable operation with zero voltageswitching (ZVS). At the output of the control path 52, the high-pressuredischarge lamp 12 is provided with an RF voltage U₀ and an RF current I₀via the abovementioned apparatus 40 for determining the actual value ofthe RF power. Particularly preferred examples of embodiments of theapparatus 40 for determining the actual value of the RF power areillustrated in FIGS. 3, 4 and 5.

FIG. 3 shows a first exemplary embodiment of an apparatus 40 fordetermining the actual value of the RF power. This embodiment functionswithout taking into consideration the phase. The RF current I₀ and theRF voltage U₀ are each independently converted into a DC voltageU_(dci), which is proportional to the RF current I₀, and U_(dcu), whichis proportional to the RF voltage U₀, by means of double peak-valuerectification. If the high-pressure discharge lamp 12, which is likewiseshown for illustrative purposes, predominantly demonstrates a resistiveresponse, this procedure is entirely sufficient for the determination ofthe power and the closed-loop control of the power. It comprises atransformer Tr1, which feeds a subcircuit which is designed fordetermining U_(dci). This subcircuit comprises a capacitor C_(ki), twodiodes D4 and D5 and a further capacitor C_(tpi). The other subcircuitis used for providing the voltage U_(dcu), which structurally has thesame design as the subcircuit mentioned first. The components used therehave the designations C_(ku), D2, D3 and C_(tpu).

FIG. 4 shows a second exemplary embodiment of an apparatus 40 formeasuring the actual value of the RF power. It is in the form of abridge mixer circuit, which, in contrast to the exemplary embodiment inFIG. 3, now takes into consideration the phase between the RF current I₀and the RF voltage U₀. It comprises a transformer Tr2, two diodes D6,D7, three capacitors C2, C3, C_(tp), an inductance L_(tp) and ameasuring resistor R_(m). At its output, the measured voltage U_(mdC) isprovided which comprises the scalar product of the RF current I₀ and theRF voltage U₀. Precise closed-loop control of the power can thereforealso be realized in the case of a non-resistive load response of thehigh-pressure discharge lamp 12.

FIG. 5 shows a third exemplary embodiment of an apparatus 40 formeasuring the actual value of the RF power. It is in the form of a ringmixer circuit and, in the same way as the exemplary embodiment in FIG.4, allows the determination of the actual value of the RF power whilsttaking into consideration the phase between the RF current and the RFvoltage. It makes it possible to determine the active power andtherefore the closed loop control of the power of nonresistive loads. Itcomprises two transformers Tr3, Tr4, four diodes D8, D9, D10, D11, acapacitor C_(tp), an inductance L_(tp) and a measuring resistor R_(m).

FIG. 6 shows the time profile of the RF current I₀ with which thehigh-pressure discharge lamp 12 is fed. The sinusoidal radiofrequencyoscillation is in this case illustrated by shading between thecontinuous envelope E. The

modulation period T is in this case 8.33 ms (120 Hz). It is repeatedcontinuously and does not change. In order to promote light contrast,three short dips are provided per modulation period T. As is indicatedat the upper edge of the graph illustrated, a modulation period T hasthree temporally successive signal sections Sa1, Sa2, Sa3, which areassociated with three different colors, namely in the exemplaryembodiment the colors green, red and blue. The mean amplitude M of theenvelope E is also shown. As can clearly be seen, the profile does nothave any commutation gaps.

FIG. 7 shows the luminous flux associated with the time profile of theRF current in FIG. 6. In FIG. 7, the average luminous flux M_(IL) isalso shown. Consideration of the time profile of the current I₀ and ofthe luminous flux I_(L) in FIGS. 6 and 7 shows considerable fluctuationsin power, for example from the region A to the region C and from theregion B to the region D.

FIG. 8 shows a second exemplary embodiment of the time profile of thecurrent I₀, which in this case comprises an RF signal and an LF signal.In this case, the envelope E of the RF signal has a varying amplitude,which is matched to the varying amplitude of the LF signal in such a waythat a constant luminous flux I_(L) results. In the illustration in FIG.8, the uppermost continuous line of the signal profile and the lowermostcontinuous line of the signal profile form the envelope E. It is obviousthat the time profile of the LF component I_(LF) changes itsmathematical sign in two successive periods.

FIG. 9 shows a third exemplary embodiment of the time profile of thecurrent I₀, which in this case, as in the exemplary embodiment in FIG.8, comprises an RF signal and an LF signal. In this case, the envelopeof the RF signal again has a constant amplitude, and the LF signal has avarying amplitude. This amplitude comprises the profile of an LF signalI_(LF) drawn in bold, with an RF signal I_(RF) of constant amplitudesuper-imposed thereon. Again the signal sections Sa1, Sa2, Sa3associated with three colors are shown.

Color wheel Current Revolution revolution amplitude Duration Durationduration No. Color sequence (relative) [μs] (relative) [μs] 1 Y1 -Yellow 87.18% 602 14.45% 4136 1 M1 - Magenta 87.18% 409 9.82% 1 G1 -Green 87.18% 567 13.61% 1 GND50 - Green dark (near dark) 41.51% 1323.17% 1 R1 - Red 113.12% 1015 24.36% 1 C1 - Cyan 87.18% 589 14.13% 1B1 - Blue 130.77% 822 19.73% 2 Y2 - Yellow 87.18% 497 11.93% 4198 2YND50 - Yellow dark (near dark) 41.51% 123 2.95% 2 M2 - Magenta 87.18%409 9.82% 2 G2 - Green 87.18% 721 17.30% 2 R2 - Red 113.12% 1033 24.79%2 C2 - Cyan 87.18% 492 11.81% 2 CND50 - Cyan dark (near dark) 40.48% 1192.86% 2 B2 - Blue 130.77% 804 19.29% 3 Y3 - Yellow 87.18% 602 14.45%4136 3 M3 - Magenta 87.18% 409 9.82% 3 G3 - Green 87.18% 567 13.61% 3GND50 - Green dark (near dark) 41.51% 132 3.17% 3 R3 - Red 113.12% 101524.36% 3 C3 - Cyan 87.18% 589 14.13% 3 B3 - Blue 130.77% 822 19.73% 4Y4 - Yellow 87.18% 497 11.93% 4198 4 YND50 - Yellow dark (near dark)41.51% 123 2.95% 4 M4 - Magenta 87.18% 409 9.82% 4 G4 - Green 87.18% 72117.30% 4 R4 - Red 113.12% 1033 24.79% 4 C4 - Cyan 87.18% 492 11.81% 4CND50 - Cyan dark (near dark) 40.48% 119 2.86% 4 B4 - Blue 130.77% 80419.29%

The above table represents an implemented sequence of light modulationin a rear-projection television which uses a six-segment color wheelwith the colors red, green, blue, yellow, magenta, cyan. The color wheelhas, as mentioned, six segments of different colors, with the differentstates, for example “Green dark” or “Yellow dark”, being generated bydimming the high-pressure discharge lamp. The frequency has fourdifferent cycles which are reproduced below

by the numbers of the color wheel revolution. The table gives therelative current amplitude, the duration in μs, the relative duration in% and the duration of the revolution of the corresponding cycle. Theoperating RF frequency is 5.6 MHz.

The scope of protection of the invention is not limited to the examplesgiven hereinabove. The invention is embodied in each novelcharacteristic and each combination of characteristics, which includesevery combination of any features which are stated in the claims, evenif this feature or combination of features is not explicitly stated inthe examples.

The invention claimed is:
 1. A circuit arrangement for operating ahigh-pressure discharge lamp with an electronic ballast, which isdesigned to provide an AC feed signal for the high-pressure dischargelamp, which AC feed signal comprises, in the time range as modulationperiod, a serial sequence of at least one first signal section, onesecond signal section and one third signal section, which are associatedwith a first, a second and a third color, wherein the AC feed signal isan amplitude-modulated RF signal with a frequency of at least 500 kHz,where the following applies for the envelope of the signal componentswith a positive and/or negative amplitude: the absolute value of theenvelope has a mean amplitude; and the first signal section and/or thesecond signal section and/or the third signal section has/have at leastone temporal range in which the absolute value of the envelope isbetween 5% and 100% greater than the mean amplitude and/or in which theabsolute value of the envelope is between 5% and 90% less than the meanamplitude.
 2. The circuit arrangement as claimed in claim 1, wherein theAC feed signal is a symmetrically amplitude-modulated RF signal, wherethe absolute value of the envelope of the signal components with apositive amplitude is equal to the absolute value of the envelope of thesignal components with a negative amplitude.
 3. The circuit arrangementas claimed in claim 1, wherein the AC feed signal comprises an RF signaland an LF signal with varying amplitude.
 4. The circuit arrangement asclaimed in claim 3, wherein the envelope of the RF signal has a constantamplitude.
 5. The circuit arrangement as claimed in claim 3, wherein theenvelope of the RF signal has a varying amplitude, where the variationis matched to the variation of the amplitude of the LF signal.
 6. Thecircuit arrangement as claimed in claim 1, wherein the absolute value ofthe envelope of at least one of the signal sections comprises a range ofa first amplitude followed by at least one range of a second amplitude,where the absolute value of the second amplitude is less than theabsolute value of the first amplitude.
 7. The circuit arrangement asclaimed in claim 6, wherein the second amplitude is from 50 to 90% ofthe first amplitude.
 8. The circuit arrangement as claimed in claim 6,wherein a range of a third amplitude is provided, with the secondamplitude being from 50 to 90%, and the third amplitude being from 2 to50% of the first amplitude.
 9. The circuit arrangement as claimed inclaim 1, wherein the electronic ballast comprises a control loop forcontrolling the RF power emitted to the high-pressure discharge lamp.10. The circuit arrangement as claimed in claim 9, wherein the controlloop comprises: an apparatus for determining the actual value of the RFpower; and a setpoint input apparatus for inputting the setpoint valuefor the RF power.
 11. The circuit arrangement as claimed in claim 10,wherein the apparatus for determining the actual value of the RF powercomprises a ring mixer or a bridge mixer.
 12. The circuit arrangement asclaimed in claim 10, wherein the apparatus for determining the actualvalue of the RF power comprises: an apparatus for fixing the RF currentemitted to the high-pressure discharge lamp; an apparatus for fixing theRF voltage present across the high-pressure discharge lamp; and anapparatus for determining the actual value of the RF power from the RFcurrent and the RF voltage.
 13. The circuit arrangement as claimed inclaim 12, wherein the apparatus for fixing the RF current emitted to thehigh-pressure discharge lamp comprises a first peak-value rectifier, andthe apparatus for fixing the RF voltage present across the high-pressuredischarge lamp comprises a second peak-value rectifier.
 14. The circuitarrangement as claimed in claim 10, further comprising afrequency-dependent load network, which provides the RF current and theRF voltage to the high-pressure discharge lamp depending on the drivingfrequency, the control loop furthermore comprising: an actuator fordetermining a change in the frequency driving the load network from thedifference between the setpoint value and the actual value of the RFpower.
 15. The circuit arrangement as claimed in claim 10, wherein thecontrol loop is designed adapted to implement the control in acolor-specific manner, i.e. separately for at least the first, thesecond and the third color.
 16. The circuit arrangement as claimed inclaim 15, wherein the setpoint input apparatus is adapted to vary thesetpoint value to be input to the control loop temporally correspondingto the present color.
 17. The circuit arrangement as claimed in claim10, wherein the setpoint input apparatus has an interface, via which atleast one setpoint value (P_(set)) can be changed.
 18. A projectionapparatus comprising a circuit arrangement as claimed in claim
 1. 19. Amethod for operating a high-pressure discharge lamp using a circuitarrangement, comprising: driving the high-pressure discharge lamp withan AC feed signal, which, in the time range as modulation period,comprises a serial sequence of at least one first signal section, onesecond signal section and one third signal section, which are associatedwith a first, a second and a third color, the AC feed signal being anamplitude-modulated RF signal with a frequency of at least 500 kHz,where the following applies for the envelope of the signal componentswith a positive and/or negative amplitude: the absolute value of theenvelope has a mean amplitude; and the first signal section and/or thesecond signal section and/or the third signal section has/have at leastone temporal range in which the absolute value of the envelope isbetween 5% and 100% greater than the mean amplitude and/or in which theabsolute value of the envelope is between 5% and 90% less than the meanamplitude.